Final answer:
The alpha subunit of a G-protein exchanges GDP for GTP upon activation by a signaling molecule, triggering a cellular response and later hydrolyzes GTP to GDP to terminate the signal.
Step-by-step explanation:
The question relates to the mechanism of G-protein-coupled receptor signaling and the role of the GDP/GTP exchange in the activation and inactivation of the G-protein's alpha (a) subunit. When a signaling molecule binds to a G-protein-coupled receptor, the alpha subunit exchanges the bound GDP for GTP, leading to the dissociation of the beta (B) and gamma (y) subunits. This exchange triggers a cellular response. Eventually, the alpha subunit will hydrolyze the GTP back to GDP, becoming inactive, and reassociate with the beta and gamma subunits, thereby terminating the signal.